Heredity and Evolution Class 10 Notes – Easy Concepts from Peas to People! (CBSE 2025

Heredity Class 10 Notes

Worried about tackling Heredity in Class 10 Science? Don’t be! This chapter doesn’t have to feel like a genetic jungle. Say goodbye to textbook confusion and complex diagrams — your ultimate guide to understanding genes, traits, and Mendel’s experiments is right here.

Crafted according to the latest NCERT 2023–24 syllabus, these easy-to-follow notes simplify every concept, from chromosomes and DNA to dominant and recessive traits.

And here’s the bonus: it’s not just about learning — it’s about making things click! Download the free PDF and turn Heredity from a tough topic into one of your strongest subjects.

From pea plants to your own DNA blueprint, this blog will help you master genetics and shine in your CBSE 2024–25 board exams. Let’s decode the science of inheritance, one concept at a time!

Subject	Science (Biology)
Class	10
Board	CBSE & State Boards
Chapter No.	8
Chapter Name	Heredity
Type	Notes
Session	2025-26

Accumulation of Variation During Reproduction

Inheritance from the previous generation provides both a common basic body design and subtle changes in it, for the next generation.
The second generation will have differences that they inherit from the first generation, as well as newly created differences.
In asexual reproduction, there is only very minor differences between them, generated due to small inaccuracies in DNA copying. However, if sexual reproduction is involved, even greater diversity will be generated.
Trait which exists in larger fraction of the population, is likely to have arisen earlier.

Heredity

Rules for the Inheritance of Traits – Mendel’s Contributions

Gene: It is the basic unit of heredity. It is a specific part (DNA segment) of a chromosome that controls the expression of a character.

Mendel chose a pea plant for his experiments because:

It is easy to grow.
It is naturally self-pollinating so is very easy to raise pure-breeding individuals.
It has a short life span as it is an annual crop so it was possible to follow several generations.
It is easy to cross-pollinate.
It has deeply contrasting characters.
The flowers are bisexual.

List of Dominant and Recessive Traits in Pea Plant

Monohybrid cross: The cross between two pea plants with one pair of contrasting characters is called a monohybrid cross.

Example: Cross between a tall and a dwarf plant.

Meldel's Experiment: Traits may be dominant or recessive

Mendel conducted a monohybrid cross with pea plants, and he observed that one of the contrasting characters disappeared in the F1 generation. This character reappears in the F2 generation (obtained by selfing F1) in just 25% of the progeny.

Mendel concluded that the character which expresses itself in F1 is the dominant character while the other one which is not able to express, though present in F1 individuals, is recessive. This recessive character is able to express only in its pure form i.e., in 25% of F2 individuals.

Dihybrid Cross: The cross between two plants having two pairs of contrasting characters is called a dihybrid cross.

Example: Cross between a round and green seed plant with a wrinkled and yellow seed plant.

Meldel's Experiment: Traits are inherited independently

Mendel carried out dihybrid crosses by crossing two pea plants differing in contrasting traits of two characters. For example, he crossed a pea plant having yellow color and round seed characters with another pea plant bearing green color and wrinkled seed characters. In the F2 generation, he obtained pea plants with two parental and two recombinant phenotypes as yellow round and green wrinkled (parental) and yellow wrinkled and green round (recombinant). This indicated that traits separated from their original parental combinations and got inherited independently.

Mendel’s Law of Inheritance

Law of Dominance: Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. The one that expresses is the dominant character and the one unexpressed is recessive.
Law of Segregation: The two members of a pair of factors separate during the formation of gametes.
Law of Independent Assortment: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair.

Genotype: The genetic makeup of an organism.

Phenotype: The observable characteristics of an organism.

How do these Traits get Expressed?

Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called the gene for that protein. Let us take the example of tallness as a characteristic. We know that plants have hormones that can trigger growth. Plant height can thus depend on the amount of a particular plant hormone. The amount of the plant hormone made will depend on the efficiency of the process for making it. Consider now an enzyme that is important for this process. If this enzyme works efficiently, a lot of hormones will be made, and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics or traits.

Chromosomes: Thread-like structures made up of DNA found in the nucleus.

Sex Determination

A human male has 22 pairs of chromosomes along with the XY sex chromosome.
A human female has 22 pairs of chromosomes along with the XX sex chromosome.
The original number of chromosomes becomes half during gamete formation. When the gamete fuse, the original number of chromosomes is restored in the progeny.

In human beings, the genes inherited from our parents decide whether we will be boys or girls. Women have a perfect pair of sex chromosomes (XX). But, men have a mismatched pair (XY).

All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus, the sex of the children will be determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.

Parents: Male (XY) x Female (XX)

Gametes: X, Y (from male) and X, X (from female)

Gametes	X	Y
X	XX (Female)	XY (Male)
X	XX (Female)	XY (Male)

🧬 Class 10 Science – Chapter 9: Heredity and Evolution

📘 NCERT Solutions (Intext + Back Exercises) | Biology | CBSE Board

🔹 Intext Questions – Page 143

Q1. If a trait A exists in 10% of a population of an asexually reproducing species and a trait B exists in 60% of the same population, which trait is likely to have arisen earlier?
✅ Answer:
Trait B is more common, so it is likely to have arisen earlier and had more time to spread through the population.

Q2. How does the creation of variations in a species promote survival?
✅ Answer:
Variations increase the chances of survival during environmental changes. Individuals with favorable variations are more likely to survive and reproduce.

🔹 Intext Questions – Page 147

Q1. How do Mendel’s experiments show that traits may be dominant or recessive?
✅ Answer:
In Mendel’s pea plant experiment, when he crossed tall and dwarf plants, the F1 generation had all tall plants, indicating tallness is dominant. The dwarf trait reappeared in F2, indicating it is recessive.

Q2. How do Mendel’s experiments show that traits are inherited independently?
✅ Answer:
Through dihybrid crosses, Mendel showed that traits like seed shape and color are inherited independently, not linked, resulting in new combinations in the F2 generation.

Q3. A man with blood group A marries a woman with blood group O, and their daughter has blood group O. Is this information enough to tell you which of the traits – blood group A or O – is dominant? Why or why not?
✅ Answer:
No, we need more data. Blood group A can be AA or AO. The daughter has OO, so both parents must have contributed O. The dominance cannot be confirmed without knowing the father's genotype.

Q4. How is the sex of the child determined in human beings?
✅ Answer:
Humans have XX (female) and XY (male) chromosomes. Females contribute X, and males contribute either X or Y. If a sperm with X fertilizes the egg, the child is female (XX); if Y, male (XY).

🔹 Intext Questions – Page 150

Q1. What are the different ways in which individuals with a particular trait may increase in a population?
✅ Answer:

Natural selection
Genetic drift
Advantageous traits increasing survival and reproduction
Mutation

Q2. Why are traits acquired during the lifetime of an individual not inherited?
✅ Answer:
Acquired traits affect somatic (body) cells, not germ cells. Inheritance occurs through DNA in germ cells only.

Q3. Why are the small numbers of surviving tigers a cause of worry from the point of view of genetics?
✅ Answer:
Small populations have less genetic variation, which reduces adaptability to environmental changes and increases the risk of genetic disorders.

🔹 Intext Questions – Page 151

Q1. What factors could lead to the rise of a new species?
✅ Answer:

Genetic variations
Natural selection
Geographical isolation
Mutation
Reproductive isolation

Q2. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or why not?
✅ Answer:
No, because self-pollinating plants reproduce within the same plant. Isolation has limited effect as gene flow is minimal anyway.

Q3. Will geographical isolation be a major factor in the speciation of an organism that reproduces sexually? Why or why not?
✅ Answer:
Yes, because geographical barriers prevent gene flow between populations, leading to divergence and eventual speciation.

🔹 NCERT Back Exercise Questions – Pages 154–155

Q1. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. The progeny all bore violet flowers, but almost half of them were short. This suggests that the genetic makeup of the tall parent plant can be depicted as:
✅ Answer: (c) TtVV
(Tall = dominant T, short = t; Violet = V, White = v)

Q2. An example of homologous organs is:
✅ Answer: (a) our arm and a dog’s fore-leg

Q3. In evolutionary terms, we have more in common with:
✅ Answer: (c) a chimpanzee

Q4. A study found that children with light-colored eyes are likely to have parents with light-colored eyes. On this basis, can we say that the light eye color trait is dominant? Why or why not?
✅ Answer:
No. A dominant trait may not always show up in every generation. We need a proper inheritance pattern or genetic cross to determine dominance.

Q5. How are the areas of study – evolution and classification – interlinked?
✅ Answer:
Classification is based on evolutionary relationships. Organisms with common ancestors are grouped together; more similar organisms have evolved more recently.

Q6. Explain the terms:
(a) analogous organs
(b) homologous organs
✅ Answer:

Analogous organs: Different origin, same function (e.g., wings of birds and insects)
Homologous organs: Same origin, different function (e.g., forelimbs of humans and bats)

Q7. Outline a project to study the evolution of a plant or animal.
✅ Answer:
Example: Study the evolution of the horse

Compare fossil records from different periods
Analyze changes in limb length, teeth shape
Study how environmental changes led to adaptations

Q8. Explain the importance of fossils in deciding evolutionary relationships.
✅ Answer:
Fossils show structural features of extinct organisms and transitional forms. They help trace lineage and understand how species evolved over time.

Q9. What evidence do we have for the origin of life from inanimate matter?
✅ Answer:
Miller-Urey experiment: simulated early Earth conditions and produced organic compounds (like amino acids), supporting chemical origin of life.

Q10. Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. Why is this advantage?
✅ Answer:
Sexual reproduction combines DNA from two parents, increasing genetic variation. This helps species adapt and survive environmental changes.

Q11. How is the equal genetic contribution of male and female parents ensured in the progeny?
✅ Answer:
Each parent contributes one set of chromosomes (23 each in humans). Gametes are haploid; fusion during fertilization restores diploid number.

🧬 Chapter 9: Heredity and Evolution

📘 Class 10 Science | CBSE Board | Previous Year Questions (PYQs)

🔹 1-Mark Questions

What do we call the type of cross in which inheritance of two traits is studied simultaneously?
📅 CBSE 2020
✅ Answer: Dihybrid cross
What is meant by heredity?
📅 CBSE 2021
✅ Answer: The passing on of traits from parents to offspring.
Name the plant on which Mendel performed his experiments.
📅 CBSE 2019
✅ Answer: Pea plant (Pisum sativum)
Which is the sex chromosome present in human males but absent in human females?
📅 CBSE 2018
✅ Answer: Y chromosome
Give an example of an acquired trait.
📅 CBSE 2017
✅ Answer: Learning to ride a bicycle

🔹 2-Mark Questions

What is the role of chromosomes in the inheritance of traits?
📅 CBSE 2023 Sample Paper
✅ Answer: Chromosomes carry genes that control traits. Each parent contributes half of the chromosomes.
Write the names of the sex chromosomes in male and female humans. Also, write the genotype of male and female.
📅 CBSE 2018
✅ Answer:

Males: XY
Females: XX

Why are acquired characters not inherited?
📅 CBSE 2016
✅ Answer: Because they do not bring changes in the DNA of germ cells.
Differentiate between dominant and recessive traits with one example each.
📅 CBSE 2019

Dominant Trait	Recessive Trait
Expressed in F1	Expressed only in homozygous
Example: Tall (T)	Example: Dwarf (t)

🔹 3-Mark Questions

In a monohybrid cross, tall pea plants were crossed with dwarf ones. The F1 generation showed all tall plants. Explain the outcome using a cross.
📅 CBSE 2020
✅ Answer:

Genotype of parents: TT × tt
F1 Genotype: Tt
F1 Phenotype: All tall (dominant trait)

Draw a diagram showing sex determination in humans.
📅 CBSE 2019
✅ Answer: Diagram showing:

Female (XX) → contributes X
Male (XY) → contributes X or Y
XX = girl, XY = boy

List three factors responsible for the rise of a new species.
📅 CBSE 2017
✅ Answer:

Genetic variation
Natural selection
Reproductive isolation

Define analogous and homologous organs with examples.
📅 CBSE 2016
✅ Answer:

Analogous: Wings of bat and insect (same function, different origin)
Homologous: Forelimbs of man and whale (same origin, different function)

🔵 5-Mark Questions

Explain Mendel’s experiment on inheritance of traits in pea plants. What were his conclusions?
📅 CBSE 2023
✅ Answer:

Crossed tall × dwarf → F1 all tall
F2 generation: 3 tall : 1 dwarf
Concluded traits are inherited in pairs as dominant and recessive

What is evolution? Describe the proof of evolution with the help of a fossil example.
📅 CBSE 2018
✅ Answer:

Gradual change over generations
Fossil example: Archaeopteryx (link between birds and reptiles)

Explain how sex is determined in human beings. Use a diagram.
📅 CBSE 2021 Sample Paper
✅ Answer:

Female: XX
Male: XY
Fertilization:
- X + X = girl
- X + Y = boy

Explain the terms variation, natural selection, and inheritance with examples. How do these help in the process of evolution?
📅 CBSE 2019
✅ Answer:

Variation: differences in traits
Natural selection: survival of the fittest
Inheritance: passing traits to offspring
These lead to gradual changes and new species

📌 Frequently Tested Concepts:

Concept	Question Type	Frequency
Mendel’s Laws (mono/di-hybrid)	3M, 5M	Very High
Sex Determination	Diagram, 3M, 5M	Very High
Acquired vs Inherited Traits	1M, 2M	High
Fossil Evidence	3M, 5M	Moderate
Homologous vs Analogous Organs	2M, 3M	High
Causes of Speciation	3M	High

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